Shulou(Shulou.com)06/01 Report--

In previous versions, we used the explain command to see the specific execution plan of SQL. A new explain extension has been added to MySQL 8.0.18, one is explain format=tree and the other is Explain Analyze based on the explain format=tree extension. Today we'll see how this command is executed.

Or use the sbtest1 table for testing.

Explain analyze select count (*) from sbtest1

This result is very strange, without where conditions, nothing can be shown, it seems to be a BUG?

OK, add a where id > 0 and try again.

Explain analyze select count (*) from sbtest1 where id > 0

Notice the red underlined section, which internally runs the query and measures the execution time.

Meaning explanation:

1) the rows=4936262 in the cost section is consistent with the result generated by explain, and both estimate the number of rows read.

2) the rows=10000000 in the actual section is the real result returned by executing this SQL, as shown in the following figure

3) actual time=62004.565 (in ms milliseconds), converted into seconds, the execution time of this SQL is 62 seconds, but there is actually a big error, you see above

The execution time is 7.98 seconds, which is rounded to 8 seconds, and the error of 62ap8 is about 77%, so we can't really believe these numbers.

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